An Directory implementation that uses the
Linux-specific O_DIRECT flag to bypass all OS level
caching. To use this you must compile
NativePosixUtil.cpp (exposes Linux-specific APIs through
JNI) for your platform.

WARNING: this code is very new and quite easily
could contain horrible bugs. For example, here's one
known issue: if you use seek in IndexOutput, and then
write more than one buffer's worth of bytes, then the
file will be wrong. Lucene does not do this (only writes
small number of bytes after seek).

WARNING: This API is experimental and might change in incompatible ways in the next release.

lockFactory - the lock factory to use, or null for the default
(NativeFSLockFactory);

forcedBufferSize - if this is 0, just use Lucene's
default buffer size; else, force this buffer size.
For best performance, force the buffer size to
something fairly large (eg 1 MB), but note that this
will eat up the JRE's direct buffer storage space